cue: fix Windows breakage

Closes #75
cuelang/cue#75

GitOrigin-RevId: ea4d1dab20c71fc1bf66f9a6077d0e19dcd0b030
Change-Id: I70736f7d67a8b37500d618573999e2fb1caad7ea
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2960
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

cue/marshal.go

@@ -165,13 +165,17 @@ func (r *Runtime) Marshal(instances ...*Instance) (b []byte, err error) {
 				dir = i.inst.Root
 			}
 			if dir != "" {
+				filename = filepath.FromSlash(filename)
 				filename, _ = filepath.Rel(dir, filename)
 				filename = filepath.ToSlash(filename)
 			}
 		}
+		// TODO: this should probably be changed upstream, but as the path
+		// is for reference purposes only, this is safe.
+		importPath := filepath.ToSlash(i.ImportPath)
 
 		staged = append(staged, instanceData{
-			Path:  i.ImportPath,
+			Path:  importPath,
 			Files: []fileData{{filename, b}},
 		})